1) It sounds to me like your bucket lid isn't getting a good seal, which allows gas to escape and not through the airlock. It happens alot.

2) Some yeasts like to flocculate quickly taking them 'out of the game' so to speak. Rousing the yeast to get them back into suspension helps. Take your fermenter and rock it slowly in a circular motion

3) Are you correcting for temperature? Is your hydrometer and/or thermometer calibrated? Check it against distilled water at the reference temperature--it should read 1.000. Some hydrometers are cheap and are off a bit. If you get any other reading than 1.000, remember the difference and adjust future readings by this offset. Check your thermometer against a glass of ice water and boiling water. It should read 32F and 212F (at sea level, adjust for altitude). Let your ice water settle for 5 minutes so the temperature stabilizes. Again, make note of any offsets for future compensation.

Thanks for the suggestions--rousing the yeast got the airlock bubbling slowly again, so I'll let it go and check it in another day or two. I'll also calibrate my instruments to make sure they aren't the culprits!

Over the past 3 days, there has been no change in the SG (still at 1.016). At this point, we have transferred to our secondary vessel with the intent of letting it bulk age for 1-2 weeks. Having come up a bit short on the final SG, is there anything to be done for this batch or is the safest thing to simply accept the result? While I understand that there may be some further fermentation over the next week, I'm not expecting anything dramatic given that the SG has been unchanged for a few days.

I think they'll be fine. If it was a really high gravity ferment and you'd been re-using the yeast for awhile, I might think differently. When I bottled I found it took 2-3 weeks to get consistency across all the bottles.